The applicability of basic concepts of modern control theory for interpreting the neuronal circuitry of cerebellar motor control has already been pointed out~L It seems legitimate to assume that the cerebeI[ar cortex is a nerve net having a high information processing ability ~ and that each piece of the cerebellar cortical sheet is linked with certain subcortical centers to form a kind of open loop feedforward control system. To develop this approach toward understanding the operation of cerebellar neuronal machinery, the vestibulo-cerebellum and its relation with the vestibuloocular reflex arc has been chosen since the neuronal construction of this system is probably the simplest among the cerebellar-controlled motor systems. The initial postulate that the cerebellum is involved in open loop control but not in closed loop control stems from a comparison between the vestibulo-ocular and vestibulo-spinal systems. The vestibulo-ocular reflex arc is a tri-neuronal chain composed of primary vestibular afferents, vestibulo-ocular relay neurons and oculomotor neurons. The vestibulo-ocular reflex arc is linked with the flocculus, i.e., the hemispheral part of the vestibulo-cerebellum, in the following way. After making excitatory synapses with the vestibulo-ocular relay neurons, tile primary vestibular afferent fibers enter the flocculus and terminate in the cerebellar cortex as a mossy fiber input. The Purkinje cells in the flocculus project in turn to the vestibular nuclei to make inhibitory synaptic contact with the vestibulo-ocular relay neurons. In this way a side path for the vestibulo-ocular reflex arc is formed, as illustrated in Fig. I. The vestibulo-spinal reflex arc is also a tri-neuronal chain, composed of the primary vestibular afferents, vestibulo-spinal relay neurons and spinal motoneurons. The ventral Deiters neurons are involved in this reflex arc 11. In cats, the ventral Deiters neurons are apparently free of Purkinje cell synapsesl, v. Recent studies on other types of vestibulo-spinal relay neurons have revealed that they receive Purkinje cell inhibition but from regions of the vermal cortex of the anterior and posterior lobes which are not directly related to the primary vestibular afferentsL Therefore, the vestibulospinal reflex arc is not equipped with a side path through the cerebellum, in contrast to the situation for the vestibulo-ocular reflex arc. The presence and absence, respectively, of a cerebellar side path can be related to the difference in the mode of operation of these two vestibular reflex arcs. The vestibulo-spinal reflex arc acts through the neck and trunk muscles to determine the position and movement of the head; this output will be readily detected by the vestibular organ. It is thus conceivable that the